Airway hyperresponsiveness (AHR), the tendency of the airways to narrow too much and too easily in response to various stimuli, is a universal feature of asthma, although it is not exclusive to this disease. Airway responsiveness shows a unimodal distribution in the general population and might vary with time, increasing after exposure to allergens, industrial substances, or infectious agents in predisposed individuals, or decreasing for variable time periods after environmental or pharmacologic interventions. Airway inflammation and structural airway changes can lead to this heightened airway response, but the mechanisms by which they modify airway function are still unclear. They might be associated with increased contractile properties of the airways--from an increase in contractile elements, a change in smooth muscle mechanical properties, or a reduction of forces opposing bronchoconstriction, such as reduced airway-parenchymal interdependence. Other factors, such as neurohumoral influences and "geometric factors" (eg, airway caliber), can modulate the degree of AHR.